TERS (Tip Enhanced Raman Spectroscopy) brings Raman spectroscopy into nanoscale resolution imaging1-6. TERS is a super-resolution chemical technique. Better yet, it is a label-free super-resolution imaging technique which has been extended by our novel technology into an important new imaging technology.
TERS imaging is performed with an AFM-Raman system, where a Scanning Probe microscope (SPM that can be used in atomic force, scanning tunneling, or normal/shear force mode) is integrated with a confocal Raman spectrometer through an opto-mechanical coupling. The scanning probe microscope allows for nanoscale imaging, the optical coupling brings the excitation laser to the functionalized tip (or probe), and the spectrometer analyzes the Raman (or otherwise scattered) light providing a hyperspectral image with nanometer scale chemical contrast.
A TERS system is based on a metallic tip (generally made of gold or silver) employed to concentrate the incident light field at the apex. The tip acts as a nano-source of light and local field enhancer, greatly improving the Raman sensitivity (by a factor of 103 -107) and reducing the probed volume to the “nano” region immediately below the tip. The optical coupling that combines the two instruments uses a confocal scheme. Two different configurations exist for this coupling: one in transmission and one in reflection (Fig. 2), having their own advantages and drawbacks.
The transmission configuration allows the use of the highest numerical aperture (NA) objectives (including immersion objectives) giving high power density at the focus point and enabling the collection of high signal level, but it can only be used for transparent samples. The reflection configuration can be used for any kind of samples (opaque and transparent) but is limited to lower NA objectives. By combining point-by-point scanning with simultaneous spectrum acquisition, near-field Raman mappings can be performed with lateral resolution down to ten nanometers or less.